1. Field of the Invention
This invention relates generally to a new method for temperature measurement that provides a direct digital temperature output, and is especially suited for low cost, low power, moderate accuracy implementation. More particularly, it relates to a measurement method for use with a sensor whose resistance or capacitance varies in a known manner with temperature. The measurement circuit is a temperature-to-digital converter which performs a zero-crossing time measurement of a step input signal that has been double differentiated using two time constants.
2. Description of Related Art
Resistive elements such as thermistors, resistive temperature devices and sensistors are routinely used to measure temperature. Two temperature measurement methods widely used with resistive sensors involve either: (1) applying a high precision current and measuring the resultant voltage with an correspondingly high precision voltage-sensitive analog-to-digital converter (ADC); or (2) applying a high precision voltage and measuring the resultant current with an equally precise current-sensitive analog-to-digital converter. Whichever measurement method is used, both require the use of not one but two functional circuit modules with approximately equal precision to produce a digital representation of temperature from the resistance-based temperature sensor. This requirement for two high precision circuit modules is a limitation of the present art heretofore unaddressed.
Temperature monitoring is often performed in situations where it is difficult to environmentally control the measurement electronics. In some automotive applications, for example, measurement electronics are required to function from -40.degree. C. to 150.degree. C. while the sensor temperature may reach combustion levels. Both aviation and oil well drilling present similarly challenging temperature measurement difficulties. Temperature measurement in situations like these where the measurement electronics and sensor temperature vary independently has been difficult. The traditional temperature measurement methods above, which rely on maintaining highly accurate and stable circuit functions over wide temperature ranges, have been at a disadvantage.
We present a new method for temperature measurement whose accuracy relies on the ability to generate an accurate timebase. A timebase function can be easily implemented in a single circuit module. Our method makes possible a moderate accuracy temperature measurement implementation using ASIC technology that can be miniaturized, battery powered, and can result in a very low cost measurement method. It eliminates the aforementioned problems associated with prior methods in that they require more circuit elements that have to be precisely controlled, and such control has been very difficult to achieve over desired temperature ranges.
Therefore, it is a first object of the present invention to provide a new temperature measurement method where the precision of measurement relies on a single high precision component in the form of a robust timebase generator.
A second object is to provide a new temperature measurement method based on the interaction of two time constants.
A third object is to provide a temperature-to-digital converter that gives direct temperature-to-digital conversion, and eliminates temperature-related errors associated with analog-to-digital conversion methods.
A fourth object is to eliminate the need for absolute voltage or current sources required in traditional sensistor, thermistor and resistive temperature device measurement circuits.
A fifth object is to eliminate the need for high gain, temperature-stable amplifiers in temperature measurements based on thermocouple sensors.
A sixth object is to provide a new temperature measurement method that is insensitive to the amplitude or gain of the signal applied to the sensor, thereby eliminating electronics drift problems associated with amplitude or gain instability.
A seventh object is to eliminate dc offset errors associated with electro-static discharge protection structures and other dc currents that become particularly significant at elevated temperatures.
An eighth object is to provide a new temperature measurement method for use with both resistance-based temperature sensors and capacitance-based temperature sensors.
Further and other objects and advantages of the present invention will become apparent from the description contained herein.